The invention relates to a method for suppressing ghost image artifacts in x-ray images, in which in chronological order, a plurality of x-ray images of one or more objects is generated with the interposition of at least one solid-state detector for generating a visible image by incident x-radiation, and from each currently generated x-ray image, a previously ascertained correction image (offset image) is electronically subtracted.
The invention also relates to an x-ray device for generating x-ray images with at least one x-ray source, one solid-state detector, and one image processing device for correcting the x-ray image taken in accordance with the above method.
Imaging x-ray devices, particularly for medical diagnosis, are widely known. In such x-ray devices, originally and in part even today, the images generated are taken directly by x-radiation on radiation-sensitive films. Over the course of development, to improve the imaging power, the films have been replaced by so-called solid-state detectors, which make digital imaging possible. These solid-state detectors are, for instance, a cesium iodide scintillator in which the x-ray quanta striking it generate light pulses, which in turn are converted by a suitably disposed photodiode matrix into electrical charge and are read out electronically. Such solid-state detectors, which can also be called direct x-ray converters, have the disadvantage that because of the property of the solid-state detector, so-called “memory” effects occur, which are due, for instance, to incomplete charge accumulation or low induced energy levels in the detector material that do not empty until later under thermal induction. As a consequence, residual signals from a previous image remain in the detector and are superimposed on an image generated later. These effects are intrinsically greater the shorter the chronological spacing between two successive images generated, or the higher the applied x-ray dose with which an image was generated.
Besides these residual-image effects, which do not occur until images are taken, solid-state detectors also have properties that vary from one pixel to another, such as dark flows, leak flows, or pixel capacitances. Also, each readout channel has various properties because of various line capacitances, input capacitances of the input amplifier, and the like. To generate an image with low noise, these last-named effects are eliminated; this is done by eliminating a correction image, which contains solely the background effects but not any image information, from an object image.
To improve the image quality of a digital x-ray image taken, it is known for instance from European Patent EP 0 642 264 B1 for a previously ascertained correction image to be subtracted electronically from an x-ray image taken, and in this way to reduce possible artifacts that arise from the specific properties of the detector matrix or from residual charges on the detector matrix.
It is intrinsically favorable on the one hand, in defining a correction image that is to be subtracted from a current image, to allow the longest possible time to elapse between a previous image and the taking of the correction image—which is done without an object—to keep the “memory” effect slight. On the other hand, however, the correction image should be as current as possible, so that changes in the detector matrix will as much as possible match the status at the instant when the image is actually taken.
Thus if a correction image, which because of the “memory” effects still contains some of an earlier image, is used to correct a new image, then a so-called “ghost image” that originates in the residues of the earlier image is generated on the corrected new image. On the other hand, a correction image that is too old, although it does not contain any “ghost image” information, is nevertheless too far away chronologically from the image to be corrected and has excessive statistical noise. This is because the correction values obtained there, because of their variation over time, no longer match the currently present correction values.